Biological Reduction of Selenium, Nitrates, Chromium, and Perchlorate

Description of the Biological Reduction Process
Biological treatment is used in a wide range of water and wastewater treatment applications for the removal of biodegradable constituents. The biological treatment process can also be modified and used to reduce nitrates, oxides of selenium, hexavalent chrome, and perchlorate in water.

The reduction of nitrates and perchlorate can be accomplished biologically through microorganisms that use an electron donor (such as methanol, acetic acid, ammonia, or hydrogen). The target compound such as nitrate and perchlorate is used as an acceptor. The electron donor and the electron acceptor are metabolized by the microorganisms to generate energy for growth and reproduction. The microbial metabolism will generate byproducts, of which the most common are carbon dioxide and water. In the biological reduction process, nitrates are reduced to nitrogen gas, perchlorates are reduced to chloride ion, and metals are reduced to a lower oxidation level.

The success of any biological processes requires adequate control of environmental conditions (i.e., food, oxygen concentration, temperature, nutrient availability). The right environmental conditions allow the reactor to select for microorganisms that perform the desired reactions. Additional parameters that assist with biogrowth include the film materials where microorganisms grow, and the method of chemical delivery.

Biological Nutrient Removal
Nitrate removal in water has been extensively accomplished through the operation of anoxic processes using suspended growth (municipal or industrial wastewater treatment) where nitrates are used as an electron acceptor, transforming nitrate into nitrogen gas. The anoxic process requires carbon source which needs to be added to sustain microorganism growth. If insufficient carbon is found in the water, it may be artificially supplied with a source like methanol. The anoxic process can be effective in reducing other compounds such as perchlorate.

Technologies using Hydrogen as an Electron Donor
This innovative technology works by using hydrogen as the electron donor, and using nitrates and perchlorate as the electron acceptors. The controlled environment is highly selective of heterotrophic microorganisms that thrive and reduce the target substances. In this process, perchlorate is reduced to chloride ion, nitrates are reduced to nitrogen gas. The process is also effective in the destruction of some VOCs.

The benefits of hydrogen based anoxic processes are: lower sludge production, and fewer undesired byproducts, low residual electron donor in the effluent that may contribute to bio-growth in downstream equipment.